In the case of an internal combustion engine having a surge tank located downstream of a throttle valve, the torque response to the operation of the accelerator pedal is determined depending on the volume of the surge tank. More specifically, as the volume of the surge tank is larger, pressure changes in the surge tank in response to changes in the throttle flow are reduced, and changes in the torque are also reduced. Namely, the torque response to the operation of the accelerator pedal slows down. On the contrary, as the surge tank volume is smaller, pressure changes in the surge tank are increased, and, therefore, the torque response speeds up.
Accordingly, if it is desired to vary the torque response to the accelerator pedal operation depending on operating conditions of the vehicle, such as the operation amount or speed of the accelerator pedal, vehicle speed, and/or the change gear ratio of the transmission, the volume of the surge tank may be changed in accordance with the operating conditions of the vehicle. However, making the surge tank volume variable renders the engine system complicated in construction, resulting in an increase of the manufacturing cost. Thus, it has been desired to provide technologies for allowing variations in the torque response to the accelerator pedal operation while keeping the surge tank volume constant, namely, without changing the surge tank volume.
For example, Japanese Laid-open Patent Publication No. H1-111543 discloses an engine control system in which the operation amount or travel of the accelerator pedal is processed by means of a first-order lag filter to provide a throttle opening, and the time constant of the first-order lag filter is varied depending on the shifting conditions of the transmission or the operation speed of the accelerator pedal. If the time constant is set to a small value, changes in the throttle opening are made highly responsive to the operation amount of the accelerator pedal, and the torque response can be enhanced. If the time constant is set to a large value, on the other hand, a delay in the response to the accelerator pedal operation is increased, and torque shock can be reduced.
However, if the throttle opening is derived from the accelerator pedal operation amount through processing with the first-order lag filter, the throttle opening does not change stepwise but changes gradually even when the accelerator pedal operation amount changes stepwise. While the torque of the internal combustion engine changes with pressure changes in the surge tank, the surge tank pressure does not change unless the throttle flow changes. Thus, with the technology as disclosed in the above-identified publication, a substantially no-response time appears during which the torque does not respond to the accelerator pedal operation, thus making it difficult or impossible to provide ideal torque response as provided in the case where the surge tank volume can be varied.